Commonly, heavier hydrocarbons known as heavy oil or bitumen are produced from subterranean formations using in-situ enhanced recovery processes that reduce the viscosity of the hydrocarbons enabling them to flow to the wells; otherwise, economic production rate would not be possible. Examples of commercial in situ enhanced recovery processes are discussed further below. When produced heavy oil or bitumen is shipped to market through pipelines without further processing, it is typically mixed with lighter less viscous oils or diluents to reduce its viscosity to meet pipeline shipping specifications. Various upgrading processes may be used to separate the crude oil into a lighter, less viscous fraction and a heavier, more viscous fraction or heavy petroleum fraction. If crude oil is upgraded on the production site, the lighter fraction is the more valuable fraction of the oil and can meet pipeline specifications without the addition of diluent or at least with reduced diluent requirement.
The heavy petroleum fraction has typically had very limited economic value due to very restricted markets. Thus, a challenge is to create value from the heavy petroleum fraction.
Commercial in situ processes to recover heavier hydrocarbons from subterranean formations exploit at least one of temperature, pressure, and a solvent to reduce the viscosity or otherwise enhance the flow of viscous hydrocarbons within the formation. Various in situ enhanced recovery processes are known including CSS (Cyclic Steam Stimulation), CSD (Constant Steam Drainage), SAGD (Steam Assisted Gravity Drainage), SA-SAGD (Solvent Assisted-Steam Assisted Gravity Drainage), VAPEX (Vapor Extraction), LASER (Liquid Addition to Steam for Enhancing Recovery), SAVEX (Combined Steam and Vapor Extraction Process), water flooding, and steam flooding.
An example of SAGD is described in U.S. Pat. No. 4,344,485 (Butler). An example of VAPEX is described in U.S. Pat. No. 5,899,274 (Frauenfeld). An example of LASER is described in U.S. Pat. No. 6,708,759 (Leaute et al.). An example of SAVEX is described in U.S. Pat. No. 6,662,872 (Gutek).
In certain processes, such as in CSS (Cyclic Steam Stimulation), the same well is used both for injecting a fluid and for producing oil. In CSS, cycles of steam injection, soak, and oil production are employed. Once the production rate falls to a given level, the well is put through another cycle of injection, soak and production.
In other processes, such as in SAGD (Steam Assisted Gravity Drainage), a dedicated injection well and a dedicated production well are used.
Another in situ enhanced recovery process is water flooding which involves injecting water into a reservoir, typically through a dedicated injection well. The water serves to displace the oil in the reservoir to a dedicated production well. However, when water flooding is applied to displace viscous heavy oil from a formation, the process is inefficient because the oil mobility is much less than the water mobility. The water quickly channels through the formation to the production well, bypassing most of the oil and leaving it unrecovered. Consequently, there is a need to either make the water more viscous, or use another drive fluid that will not channel through the oil. Because of the large volumes of drive fluid needed, it should be inexpensive and stable under formation flow conditions. For moderately viscous oils (i.e. those having viscosities of approximately 20 to 100 cP), water-soluble polymers have been used to increase the viscosity of the water injected to displace oil from the formation. Water and oil macroemulsions have also been used as drive fluids, and an example of such a process is disclosed in Canadian Patent Application No. 2,405,493 (Bragg et al., published Nov. 1, 2001). The type of process described in that patent application is described as an enhanced oil recovery (EOR) procedure since a significant percentage of oil remains in a subterranean formation after the costs of primary production rise to such an extent that further oil recovery is cost-ineffective.
Another in-situ recovery process similar to water flooding is steam flooding. In steam flooding, steam is injected into the injection well instead of water. Typically, most of the steam condenses in the reservoir so that oil, water and a small fraction of steam is produced from the production well. The heat of the steam reduces the oil viscosity and enables higher heavy oil production rates. Steam flooding of heavy oil, like water flooding of heavy oil, is subject to the challenge that the steam or water tends to finger through to the production well.
In the art, processes using a drive fluid are often referred to as displacement processes. In one prior example, water and oil macroemulsions are used as drive fluids (Canadian Patent Application No. 2,405,493 (Bragg et al., published Nov. 1, 2001)).
Turning now to a known use of a heavy petroleum fraction, U.S. Pat. No. 4,113,013 (Ledoux), issued Sep. 12, 1978, describes a process for producing petroleum from subterranean formations wherein production from the formation is obtained by driving a fluid from an injection well to a production well. The process involves injecting into the formation, via the injection well, asphaltenes as sacrificial agents to inhibit the deposition of chemical recovery agents such as surfactant on the reservoir matrix. The patent describes that normally the process would be carried out by first injecting the asphaltenes into the formation through the injection well and following them with a chemical recovery agent. The chemical recovery agent would then be followed by a drive fluid such as water to push the materials to the production well. The injected asphaltenes may be present in crude oil, may be in solution in a solvent, or may be in a crude oil diluted with solvent. This patent does not describe the use of a heavy petroleum fraction as a drive fluid in the recovery of hydrocarbons from a subterranean formation.
U.S. Pat. No. 4,550,779 (Zakiewicz), issued Nov. 5, 1985, describes a process for recovering hydrocarbons, and especially heavy hydrocarbons, wherein mining liquids are injected at a bottom level and removed at an upper collecting level together with displaced hydrocarbons. The mining liquids are separated from the product hydrocarbons at the well end and re-injected. The mining liquids are light fractions of product cracking or semi-refining carried out in the vicinity of the wellhead, together with inorganic solvents that are injected under pulsating pressure at a temperature not exceeding 100° C. The mining fluids reduce the viscosity of the heavy hydrocarbons. This patent does not describe the use of a heavy petroleum fraction as a drive fluid in the recovery of hydrocarbons from a subterranean formation.